1. Field of the Invention
The present invention relates to automatic gain control circuits and, more particularly, to a gain control circuit useful in a position encoder.
2. Description of the Prior Art
In recent years, optical encoders have been developed employing a combination of a light source and light detectors, which are separated by an apparatus on a rotatable shaft for interrupting the light in predetermined fashion as the shaft is moved. Optical encoders are typically employed to detect and signal the angular position and velocity of a rotating shaft and are especially useful for systems that control the position and velocity of carriages and print wheels in printers.
An incremental encoder disc contains a plurality of alternating clear and opaque sectors in a predetermined circumferential pattern which rotates with the shaft. The disc may cooperate with a suitable masking device to limit the light that impinges upon a photosensitive transducer to signal the passage of the sectors.
With encoders that must be fitted in a small area and where a premium is placed upon a compact package, designers have turned to photo emissive devices such light emitting diodes, operating in conjunction with a plurality of photodetectors that may be part of a single semiconductor substrate. For example, a commercially successful device which utilizes a silicon chip having a single diffusion layer with several separate photocells formed thereon is employed in an optical encoder produced by the assignee of the present invention, which is described in the recent patent to Weber, U.S. Pat. No. 4,224,514, issued Sept. 23, 1980.
In that patent, a gain control circuit was described which relied upon a dedicated photodetector that viewed an uninterrupted portion of the light beam from the light source. A feedback circuit was then devised which regulated the voltage to the light emitting diode to maintain a certaian desired signal level from the photo detector.
A similar concept was disclosed in the patent to Grundherr, U.S. Pat. No. 4,118,129, issued Oct. 3, 1978 for a Rotary Wheel Printing System. In this apparatus a pair of series connected LED's was controlled by a separate photocell which received radiant energy directly from both LED's, and which was used in a feedback circuit to control the voltage and therefore the brightness of the LED elements.
While the optical encoder of Weber has proven to be commercially successful, it has been deemed desirable to reduce the area that must be illuminated by the light emitting diode, and to concentrate more of the illumination on the phototransducers that are in the direct path of the optical encoder disc. If the separate feed-back photo detector could be eliminated then the LED and its lens could be redesigned to direct substantially all of the illumination to the main photo detectors, which Weber utilized to obtain position and velocity information.
Weber teaches two pairs of detectors in a quadrature arrangement so that the two detectors of one pair are respectively exposed and occluded simultaneously with respect to the light source. The detectors of the other pair, which are effectively phased to be 90 degrees from the first pair, are both partially illuminated. As the code disc revolves, of the two apertures that were partially obscured, one becomes completely obscured while the other is completely open. Of the other pair, both become partially obscured and therefore partially illuminated.
The net result is, that of the four photo detectors, at each increment of movement, only one sensor is totally obscured, one sensor is completely exposed and the remaining two sensors are half obscured. If one assumes that the total available light flux for one of the photo detectors is X, then four detectors will, at all times, receive a light flux of 2X, shared among the sensors. As the encoder and disc rotate, the distribution of light flux varies, but the net illumination at any instant in time, in this embodiment, remains 2X.
If the magnitude of the photo detector signal remains constant and is sufficiently above the noise level, then subsequent amplification stages can bring the output of the encoder to any desired level. The embodiments described do not materially affect the amplitude of the waveform.